The invention is directed to a measuring arrangement for the determination of the activity or concentration of ions in solution which comprises at least one measuring electrode and a reference electrode with which are connected by a flow channel, provided with a first exit and a second exit, which is connected with at least one calibration container by means of a dosage line provide with a first pump and at least one valve, wherein said dosage line is further provided on exit line between said first pump and said flow channel in which there is provided a second pump. The test sample line measuring arrangement is connected with said flow channel.
An arrangement of the foregoing type is disclosed in DE-OS 3416956 (U.S. Pat. No. 4,797,191), the disclosure of which is incorporated herein by reference. This prior art device comprises an electrode unit (ion sensitive electrode and reference electrode) and a flow channel connected with this electrode unit. The first exit 23 of the flow channel is connected over a first line with the calibration solution containers, the exit is provided with a sampling guide line. In this first line, there is provided a pump which forwards the calibration solution to the electrode entity. From this first line, a discharge or waste line branches off to the discharge container. At both exits of the flow channel, downstream of the first pump, as well as in the discharge line, there are provided flow through sensors with which the fill level of the respective lines are tested. Downstream of the calibration container and before the discharge container, a blocking valve is provided. In the discharge line, there is provided a second pump which is suitably combined with the first pump to yield a double sided pump with two pumping segments. With such an arrangement as described above, the sample is measured and thereafter the calibration carried out. The electrodes must be washed before and after calibration in order to avoid errors due to residues of the sample or calibration fluid.
In order to carry out the washing of the flow channel, the calibration fluid is pumped by the pump through the second exit in the flow channel till this channel is completely filled. In this stage, test residues which were not entirely pumped out during the measuring step can be forced into the sample guide line and from there arrive in the suction aspiration cannula from which these sample residues could drip back into the sample container.
After the washing step, the calibration fluid is pumped off through the second exit in the flow channel and, via the discharge line, is discharged into the discharge container. There is no washing of the sample guide line or the aspiration cannula. In order to avoid sample contamination, it is therefore desirable to change the test sample aspiration cannula before the next measurement.
The purpose of the present invention therefore is to provide an arrangement such as that described hereinabove, which as been modified so that the leakage of the sample fluid from the aspiration cannula is avoided and that a faster and more intensive cleaning of the flow channel is achieved.